Group iii nitride composite substrate and method for manufacturing the same, and method for manufacturing group iii nitride semiconductor device

ABSTRACT

A group III nitride composite substrate includes a group III nitride film and a support substrate formed from a material different in chemical composition from the group III nitride film. The group III nitride film has a thickness of 10 μm or more. A sheet resistance of a-group III-nitride-film-side main surface of the group III nitride composite substrate is 200 Ω/sq or less. A method for manufacturing a group III nitride composite substrate includes the steps of bonding the group III nitride film and the support substrate to each other; and reducing the thickness of at least one of the group III nitride film and the support substrate bonded to each other. Accordingly, a group III nitride composite substrate of a low sheet resistance that is obtained with a high yield as well as a method for manufacturing the same are provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a group III nitride composite substrateand a method for manufacturing the same, as well as a method formanufacturing a group III nitride semiconductor device using the groupIII nitride composite substrate.

2. Description of the Background Art

Group III nitride semiconductors such as GaN, AlN, Al_(x)Ga_(1-x)N(0<x<1), and the like have superior semiconductor properties and aretherefore suitable for a substrate of a semiconductor device. Such groupIII nitride semiconductors are expensive. Therefore, in order to reducethe cost of manufacturing a semiconductor device, there has beenproposed a substrate of a semiconductor device, specifically asemiconductor substrate in which a film of a group III nitridesemiconductor such as GaN, AlN, or the like is formed on a supportsubstrate such as silicon substrate.

For example, Japanese Patent Laying-Open No. 2006-210660 discloses amethod for manufacturing a semiconductor substrate including the stepsof: implanting ions into a front surface and its vicinity of a firstnitride semiconductor substrate made from GaN, AlN, or the like;superposing a second substrate on the front surface of the first nitridesemiconductor substrate; heat-treating the two substrates superposed oneach other; and stripping most of the first nitride semiconductorsubstrate from the second substrate along a layer in which the ions areimplanted.

SUMMARY OF THE INVENTION

In a semiconductor substrate produced by the method for manufacturing asemiconductor substrate disclosed in Japanese Patent Laying-Open No.2006-210660, the thickness of the nitride semiconductor formed on thesupport substrate is a thin thickness on the order of 2 μm, and thus thesemiconductor substrate suffers from a problem of a high sheetresistance of the substrate, and a problem of a low yield because thesubstrate is partially of an increased resistance due to damage causedby the ion implantation.

The present invention aims to solve the above problems and provide agroup III nitride composite substrate having a low sheet resistance andproduced with a high yield, and a method for manufacturing the same, aswell as a method for manufacturing a group III nitride semiconductordevice using the group III nitride composite substrate.

According to an aspect of the present invention, a group III nitridecomposite substrate includes a group III nitride film and a supportsubstrate formed from a material different in chemical composition fromthe group III nitride film. Here, the group III nitride film is joinedto the support substrate in one of a direct manner and an indirectmanner. The group III nitride film has a thickness of 10 μm or more. Asheet resistance of a group II-nitride-film-side main surface of thegroup III nitride composite substrate is 200 Ω/sq or less. Here, thegroup III-nitride-film-side main surface is the main surface which iscommon to the group III nitride composite substrate and the group IIInitride film. In this group III nitride composite substrate, thethickness of the group III nitride film is 10 μm or more, and therefore,the sheet resistance of the group III-nitride-film-side main surface ofthe group III nitride composite substrate can be reduced to 200 Ω/sq orless, and group III nitride semiconductor devices can thus bemanufactured with a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, an area of a joined region joining thegroup III nitride film and the support substrate is 70% or more relativeto an area of the main surface, a non-joined region failing to join thegroup III nitride film and the support substrate includes at least onenon-joined partial region, and the non-joined partial region may be asmall non-joined partial region having a maximum size in radialdirection of less than 20 mm. In this group III nitride compositesubstrate, the area of the joined region is a large area of 70% or morerelative to the area of the main surface and the maximum size in radialdirection of the non-joined partial region forming the non-joined regionis a small size of less than 20 mm. Therefore, group III nitridesemiconductor devices can be manufactured with a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, a non-joined region failing to join thegroup III nitride film and the support substrate includes at least onenon-joined partial region, and the non-joined partial region may be aninner non-joined partial region failing to abut on a perimeter of themain surface. In this group III nitride composite substrate, thenon-joined partial region forming the non-joined region does not abut onthe perimeter of the main surface. Therefore, group III nitridesemiconductor devices can be manufactured with a high yield

In the group III nitride composite substrate according to the aboveaspect of the present invention, the group III nitride film has amain-surface through hole, and an area of the main-surface through holemay be 10% or less relative to an area of the main surface. In thisgroup III nitride composite substrate, the area of the main-surfacethrough hole is 10% or less relative to the area of the main surface.Therefore, group III nitride semiconductor devices can be manufacturedwith a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, a joint interface between the group IIInitride film and the support substrate includes an impurity containingmetal, and the concentration of the impurity may be 1×10¹⁰ cm⁻² or more.In this group III nitride composite substrate, the concentration of theimpurity containing metal in the joint interface is 1×10¹⁰ cm⁻² or more.Therefore, group III nitride semiconductor devices with a high jointstrength can be manufactured with a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, the group III nitride film may have athermal expansion coefficient of more than 0.7 times and less than 1.4times as large as a thermal expansion coefficient of the supportsubstrate. In this group III nitride composite substrate, the thermalexpansion coefficient of the group III nitride film is more than 0.7times and less than 1.4 times as large as the thermal expansioncoefficient of the support substrate. Therefore, in the step of applyingheat during manufacture of a group III nitride semiconductor device,occurrence of warp and/or cracks is prevented, and accordingly group IIInitride semiconductor devices can be manufactured with a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, the support substrate may have afracture toughness of 1 MNm^(−2/3) or more, and the support substratemay have a thickness of 50 μm or more. In this group III nitridecomposite substrate, the mechanical strength is high. Therefore, groupIII nitride semiconductor devices can be manufactured with a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, the indirect manner may be a manner ofinterposing a joint film between the group III nitride film and thesupport substrate. In this group III nitride composite substrate, thegroup III nitride film and the support substrate are joined to eachother with the joint film interposed therebetween. Therefore, group IIInitride semiconductor devices having a high joint strength can bemanufactured with a high yield.

A method for manufacturing a group III nitride composite substrateaccording to another aspect of the present invention is a method formanufacturing a group I nitride composite substrate of the above aspect,and includes the steps of: bonding the group III nitride film and thesupport substrate to each other in one of a direct manner and anindirect manner; and reducing the thickness of at least one of the groupII nitride film and the support substrate bonded to each other. Thismethod for manufacturing a group III nitride composite substrateincludes these steps so that group III nitride composite substrates witha low sheet resistance can be manufactured with a high yield.

A method for manufacturing a group ITT nitride semiconductor deviceaccording to still another aspect of the present invention is a methodfor manufacturing a group III nitride semiconductor device using a groupIII nitride composite substrate of the above aspect, and includes thesteps of: preparing the group III nitride composite substrate; andgrowing at least one group III nitride layer on the main surface of theside of the group III nitride film of the group III nitride compositesubstrate. This method for manufacturing a group III nitridesemiconductor device includes these steps so that group III nitridesemiconductor devices can be manufactured with a high yield.

In accordance with the present invention, a group III nitride compositesubstrate having a low sheet resistance and produced with a high yield,and a method for manufacturing the same, as well as a method formanufacturing a group III nitride semiconductor device using the groupIII nitride composite substrate can be provided.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an example of thegroup III nitride composite substrate according to the presentinvention.

FIG. 2 is a schematic cross-sectional view showing another example ofthe group III nitride composite substrate according to the presentinvention.

FIG. 3 is a schematic plan view showing a joined region and non-joinedregions in the group III nitride composite substrate according to thepresent invention.

FIG. 4 is a schematic cross-sectional view showing an example of themethod for manufacturing a group III nitride composite substrateaccording to the present invention.

FIG. 5 is a schematic cross-sectional view showing an example of thegroup III nitride semiconductor device produced by the method formanufacturing a group III nitride semiconductor device according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment Group IIINitride Composite Substrate

Referring to FIGS. 1 and 2, a group III nitride composite substrate 1which is an embodiment of the present invention includes a group IIInitride film 13 and a support substrate 11 formed from a material whichis different in chemical composition from group III nitride film 13.Here, group III nitride film 13 is joined to support substrate 11 in oneof a direct manner and an indirect manner. Group III nitride film 13 hasa thickness of 10 μm or more. A sheet resistance of a groupII-nitride-film 13-side main surface 13 m of group III nitride compositesubstrate 1 is 200 Ω/sq or less. Here, group III-nitride-film-side mainsurface 13 m is the main surface which is common to group III nitridecomposite substrate 1 and group III nitride film 13.

In group III nitride composite substrate 1 of the present embodiment,group III nitride film 13 has a thickness of 10 μm or more, andtherefore, the sheet resistance of group III-nitride-film 13-side mainsurface 13 m can be reduced to 200 Ω/sq or less. Thus, group III nitridesemiconductor devices can be manufactured with a high yield. Detailswill be described in the following.

Group III nitride composite substrate 1 of the present embodimentincludes group III nitride film 13, and support substrate 11 formed froma material different in chemical composition from group III nitride film13.

<Group III Nitride Film>

Group III nitride film 13 is a substrate formed from a semiconductorwhich is a compound of at least one group ITT element and nitrogen, andthe substrate may for example be GaN substrate, AlN substrate,Al_(x)Ga_(1-x)N substrate (0<x<1), or the like. The method formanufacturing this group III nitride film 13 is not particularlylimited. This method may be a gas phase method such as HVPE (HydrideVapor Phase Epitaxy), sublimation method, or the like, may be a liquidphase method such as high nitrogen pressure solution method, fluxmethod, or the like. In order to grow a group III nitride layer of highcrystal quality on group III nitride film 13, group III nitride film 13is preferably crystal, and more preferably single crystal.

In order to reduce the sheet resistance of group III nitride film 13,group III nitride film 13 may be doped with a conductivity improverimpurity. This conductivity improver impurity is not particularlylimited. O (oxygen) atom, Si (silicon) atom, and the like are suitablesince they are highly effective in improving the conductivity.

<Support Substrate>

Support substrate 11 is a substrate supporting group III nitride film13, and is not particularly limited as long as it is a substrate formedfrom a material which is different in chemical composition from groupIII nitride film 13. The support substrate may be an oxide substrate,and examples of the oxide substrate are: sapphire substrate and otherAl₂O₃ substrates; mullite substrate and other Al₂O₃—SiO₂-basedsubstrates; spinel substrate and other Al₂O₃—MgO-based substrates;Al₂O₃—SiO₂—YSZ (Yttria Stabilized Zirconia)-based substrate, and thelike. The support substrate may also be a metal substrate such as Mosubstrate, W substrate, Cu—W substrate, or the like. It may also be Sisubstrate. SiC substrate, graphite substrate, or the like. In addition,in the case where group III nitride film 13 is a GaN film, the supportsubstrate may be a substrate formed from AlN for example which is agroup III nitride different in chemical composition from GaN. Supportsubstrate 11 may be either crystal or amorphous. In the case wheresupport substrate 11 is crystal, it may be either single crystal orpolycrystal.

<Manner of Joining Group III Nitride Film and Support Substrate>

In group III nitride composite substrate 1 of the present embodiment,group III nitride film 13 is joined to support substrate 11 in one of adirect manner and an indirect manner.

Referring to FIG. 1, the joint in a direct manner means that group IIInitride film 13 and support substrate 11 are directly joined to eachother without a separate element interposed therebetween. In the case ofthis direct joint manner, a joint interface 100 is an interface formedby a main surface 13 n of group III nitride film 13 and a main surface11 m of support substrate 11 that are joined to each other.

Referring to FIG. 2, the joint in an indirect manner means that groupIII nitride film 13 and support substrate 11 are indirectly joined toeach other with a separate element such as a joint film 12 for exampleinterposed therebetween. In the case of this indirect joint manner, thejoint interface varies depending on how the substrate and the film arebonded to each other as described below.

A first bonding method is as follows. As shown in FIGS. 2 and 4, a jointfilm 12 a is formed on main surface 11 m of support substrate 11 and ajoint film 12 b is formed on main surface 13 n of group III nitride film13. After this, a main surface 12 am of joint film 12 a and a mainsurface 12 bn of joint film 12 b are bonded to each other to therebyform joint film 12 into which joint film 12 a and joint film 12 b areintegrated. In this case, joint interface 100 is a surface formed bymain surface 12 am of joint film 12 a and main surface 12 bn of jointfilm 12 b that are joined to each other in joint film 12.

A second bonding method is as follows. Joint film 12 is formed on mainsurface 11 m of support substrate 11 and thereafter the main surface ofjoint film 12 and main surface 13 n of group III nitride film 13 arebonded to each other. In this case, joint interface 100 is a surfaceformed by the main surface of joint film 12 and main surface 13 n ofgroup III nitride film 13 that are joined to each other.

A third bonding method is as follows. Joint film 12 is formed on mainsurface 13 n of group III nitride film 13, and thereafter the mainsurface of joint film 12 and main surface 11 m of support substrate 11are bonded to each other. In this case, joint interface 100 is a surfaceformed by the main surface of joint film 12 and main surface 11 m ofsupport substrate 11 that are joined to each other.

<Joint Film>

Joint film 12 which may be included between group III nitride film 13and support substrate 11 in group III nitride composite substrate 1 ofthe present embodiment is not particularly limited as long as the jointfilm serves to increase the strength with which group III nitride film13 and support substrate 11 are joined. The joint film may be SiO₂ film,Si₃N₄ film, AlN film, Al₂O₃ film, TiO₂ film, TiN film, Ga₂O₃ film, Wfilm, Mo film, Au—Sn film, or the like.

The thickness of joint film 12 is not particularly limited. For the sakeof increasing the strength with which group III nitride film 13 andsupport substrate 11 are joined to each other, the thickness ispreferably 0.05 μm or more, and more preferably 0.1 μm or more. For thesake of improving the in-plane uniformity and the flatness of joint film12, the thickness is preferably 5 μm or less, and more preferably 2 μmor less.

<Thickness of Group III Nitride Film>

Regarding group III nitride composite substrate 1 in the presentembodiment, for the sake of reducing the sheet resistance of groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1, the thickness of group III nitride film 13 needsto be 10 μm or more, and is preferably 50 μm or more, and morepreferably 100 μm or more. For the sake of reducing the cost of groupIII nitride composite substrate 1, the thickness of group III nitridefilm 13 is preferably 500 μm or less, and more preferably 250 μm orless.

<Sheet Resistance of Group III-Nitride-Film-Side Main Surface>

Regarding group III nitride composite substrate 1 in the presentembodiment, for the sake of improving device properties of a group IIInitride semiconductor device (such as luminous efficiency of alight-emitting device, for example) that are achieved by reducing thesheet resistance of group III nitride composite substrate 1, the sheetresistance of group III-nitride-film 13-side main surface 13 m of groupIII nitride composite substrate 1 needs to be 200 Ω/sq (ohms per square)or less, and is preferably 50 Ω/sq or less, and more preferably 10 Ω/sqor less.

<Joined Region Joining Group III Nitride Film and Support Substrate andNon-Joined Region Failing to Join Group III Nitride Film and SupportSubstrate>

Referring to FIGS. 1 to 3, in group III nitride composite substrate 1 ofthe present embodiment, group III nitride film 13 is bonded to supportsubstrate 11 along the above-described joint interface 100. Like a groupIII nitride composite substrate produced by the ion implantation method,group III nitride composite substrate 1 in the present embodiment alsohas surface roughness of respective main surfaces of support substrate11, group III nitride film 13, and joint films 12, 12 a, 12 b, amain-surface through hole 13 h of group III nitride film 13,non-uniformity of bonding, and the like. As a result, in group IIInitride composite substrate 1 of the present embodiment, joint interface100 includes a joined region 100 b where group III nitride film 13 andsupport substrate 11 are joined to each other in either the directmanner or the indirect manner, and a non-joined region 100 n where groupIII nitride film 13 and support substrate 11 are not joined to eachother in either the direct manner or the indirect manner.

The presence, respective positions, respective sizes, and respectiveareas of joined region 100 b and non-joined region 100 n in jointinterface 100 of group III nitride composite substrate 1 in the presentembodiment can be measured by means of an ultrasonic microscope, adefect evaluation apparatus, or the like.

Non-joined region 100 n in joint interface 100 of group III nitridecomposite substrate 1 in the present embodiment includes at least onenon-joined partial region 111 n, 112 n, 121 n, 122 n. Although the shapeof non-joined partial regions 111 n, 112 n, 121 n, 122 n is indefinite,the shape is substantially circular or elliptical. Therefore, the sizeof the non-joined region is evaluated on the basis of the maximum sizein radial direction (maximum diameter). Non-joined partial regions 111n, 12 n, 121 n, 122 n can be classified by size into small non-joinedpartial regions 111 n, 112 n having a maximum size in radial directionof less than 20 mm, and large non-joined partial regions 121 n. 122 nhaving a maximum radial size of 20 mm or more. Non-joined partialregions 111 n, 112 n, 121 n, 122 n can also be classified, by theposition where the region is located, into inner non-joined partialregions 111 n, 121 n which do not abut on a perimeter 1 r of a mainsurface 1 m, and outer non-joined partial regions 112 n, 122 n whichabut on perimeter 1 r of main surface 1 m.

Regarding group III nitride composite substrate 1 in the presentembodiment, in order to manufacture group III nitride semiconductordevices with a high yield, the area of joined region 100 b joining groupIII nitride film 13 and support substrate 11 relative to the area ofmain surface 1 m is preferably 70% or more, and more preferably 85% ormore. If the area of joined region 100 b joining group III nitride film13 and support substrate 11 relative to the area of main surface 1 m isless than 70%, group III nitride film 13 and support substrate 11 aremore likely to be separated from each other during a process ofmanufacturing a group III nitride semiconductor device, which makes itdifficult to increase the yield of group III nitride semiconductordevices.

Regarding group III nitride composite substrate 1 in the presentembodiment, in order to manufacture group III nitride semiconductordevices with a high yield, the non-joined partial region is preferablysmall non-joined partial region 111 n, 112 n having a maximum size inradial direction of less than 20 mm. Namely, in FIG. 3, any non-joinedpartial region is preferably small non-joined partial region 111 n, 112n.

Moreover, regarding group III nitride composite substrate 1 in thepresent embodiment, in order to manufacture group III nitridesemiconductor devices with a high yield, the non-joined partial regionis preferably inner non-joined partial region 111 n, 121 n which doesnot abut on perimeter 1 r of main surface 1 m. Namely, in FIG. 3, anynon-joined partial region is preferably inner non-joined partial region111 n, 121 n. In the case of group III nitride composite substrate 1including outer non-joined partial region 112 n, 122 n, contaminants mayenter joint interface 100 during a process of fabricating group IIInitride composite substrate 1 and a process of manufacturing a group IIInitride semiconductor device using group III nitride composite substrate1, and the contaminants are difficult to remove by cleaning. In thiscase, the yield of group III nitride semiconductor devices is difficultto increase. In addition, group III nitride composite substrate 1including outer non-joined partial region 112 n, 122 n is broken throughtreatment during the step of reducing the thickness of at least one ofthe bonded group III nitride film 13 and support substrate 11. In thiscase as well, the yield of group III nitride semiconductor devices isdifficult to increase.

Regarding group III nitride composite substrate 1 in the presentembodiment, in order to manufacture group III nitride semiconductordevices with a high yield, the non-joined partial region is morepreferably small and inner non-joined partial region 111 n. Namely, inFIG. 3, any non-joined partial region is preferably small and innernon-joined partial region 111 n.

<Main-Surface Through Hole of Group III Nitride Film>

Referring to FIGS. 1 to 3, regarding group III nitride compositesubstrate 1 in the present embodiment, in order to manufacture group IIInitride semiconductor devices with a high yield, the area ofmain-surface through hole 13 h of group III nitride film 13 relative tothe area of main surface 1 m, 13 m is preferably 10% or less, and morepreferably 5% or less. Here, main-surface through hole 13 h is a holeextending through group III nitride film 13 between its main surfacewhich is directly or indirectly joined to support substrate 11 and theother surface (specifically another main surface and side surface).

The presence of main-surface through hole 13 h of group III nitride film13 in group III nitride composite substrate 1 causes non-joined region100 n to be formed on main-surface through hole 13 h and its nearbyregion. When a group III nitride semiconductor device is cleaned in theprocess of manufacturing the group III nitride semiconductor device, acleaning agent may enter the joint interface through main-surfacethrough hole 13 h to thereby cause reduction of the joint strength ofgroup III nitride composite substrate 1. If the area of main-surfacethrough hole 13 h relative to the area of main surface 1 m is 10% ormore, the area of non-joined region 100 n is accordingly larger,resulting in the difficulty in increasing the yield of group III nitridesemiconductor devices.

<Impurity Containing Metal in Joint Interface>

Referring to FIGS. 1 to 3, regarding group III nitride compositesubstrate 1 in the present embodiment, in order to manufacture group IIInitride semiconductor devices with a high yield by increasing the jointstrength, the concentration of an impurity containing metal that isincluded in joint interface 100 between group III nitride film 13 andsupport substrate 11 is preferably 1×10¹⁰ cm⁻² or more, and morepreferably 1.5×10¹⁰ cm⁻² or more. The impurity containing metal is notparticularly limited. In order to increase the joint strength, theimpurity is preferably an oxide of a base metal such as Fe (iron), Ni(nickel), or the like which has a higher ionization tendency and moreeasily oxidizes than H (hydrogen), in the case where group III nitridecomposite substrate 1 includes, as joint film 12, an oxide film such asSiO₂ film.

<Thermal Expansion Coefficient of Group III Nitride Film and ThermalExpansion Coefficient of Support Substrate>

Referring to FIGS. 1 to 3, regarding group III nitride compositesubstrate 1 in the present embodiment, in order to suppress occurrenceof warp and/or cracks during manufacture of a group III nitridesemiconductor device and thereby manufacture group III nitridesemiconductor devices with a high yield, group III nitride film 13 has athermal expansion coefficient of preferably more than 0.7 times and lessthan 1.4 times, and more preferably 0.75 times or more and 1.25 times orless as large as a thermal expansion coefficient of support substrate11.

<Fracture Toughness and Thickness of Support Substrate>

Referring to FIGS. 1 to 3, regarding group III nitride compositesubstrate 1 in the present embodiment, in order to suppress occurrenceof warp and/or cracks during manufacture of a group III nitridesemiconductor device, the support substrate has a fracture toughness ofpreferably 1 MNm⁻²′ or more, and more preferably 1.5 MNm^(−2/3) or more.In addition, the support substrate has a thickness of preferably 50 μmor more, and more preferably 100 μm or more.

Second Embodiment Method for Manufacturing Group III Nitride CompositeSubstrate

Referring to FIGS. 1, 2, and 4, a method for manufacturing group IIInitride composite substrate 1 which is another embodiment of the presentinvention is a method for manufacturing group III nitride compositesubstrate 1 in the first embodiment, and includes the steps of: bondinggroup III nitride film 13 and support substrate 11 to each other in oneof a direct manner and an indirect manner (FIG. 4 (A)); and reducing thethickness of at least one of group III nitride film 13 and supportsubstrate 11 (FIG. 4 (B)). The method for manufacturing group IIInitride composite substrate 1 in the present embodiment can include thesteps above to thereby manufacture group III nitride compositesubstrates of a low sheet resistance with a high yield.

<Step of Bonding Group III Nitride Film and Support Substrate>

Referring to FIGS. 1, 2, and 4, the method for manufacturing group IIInitride composite substrate 1 in the present embodiment includes thestep of firstly bonding group III nitride film 13 and support substrate11 to each other in one of a direct manner and an indirect manner. Here,bonding group III nitride film 13 and support substrate 11 to each otherin a direct manner means that group III nitride film 13 and supportsubstrate 11 are directly bonded to each other without a separateelement interposed therebetween. Bonding group III nitride film 13 andsupport substrate 11 to each other in an indirect manner means thatgroup III nitride film 13 and support substrate 11 are indirectly bondedto each other with a separate element such as joint film 12 interposedtherebetween.

The method for bonding group III nitride film 13 and support substrate11 to each other is not particularly limited regardless of whether theyare bonded in the direct manner or the indirect manner. Suitable methodsinclude, a direct bonding method according to which respective surfacesto be bonded to each other are cleaned, bonded together as they are, andthereafter heated to approximately 600° C. to 1200° C. so that they arejoined together; a surface activated bonding method according to whichthe surfaces to be bonded to each other are cleaned, subjected to anactivation treatment by means of plasma or ions, and thereafter joinedtogether in a low-temperature atmosphere of room temperature (25° C. forexample) to 400° C.; a high-pressure bonding method according to whichthe surfaces to be bonded to each other are cleaned with a chemicalsolution and pure water and thereafter subjected to a high pressure onthe order of 0.1 MPa to 10 MPa so that they are joined together; a highvacuum bonding method according to which the surfaces to be bonded toeach other are cleaned with a chemical solution and pure water, andthereafter joined together in a high vacuum atmosphere on the order of10⁻⁶ Pa to 10⁻³ Pa, and the like. Any of the above-referenced bondingmethods can increase the temperature to approximately 600° C. to 1200°C. after they are joined together to thereby further increase the jointstrength. In particular, the surface activated bonding method, the highpressure bonding method, and the high vacuum bonding method exhibit ahigher effect of increasing the joint strength through the heating toapproximately 600° C. to 1200° C. after they are joined together.

While group III nitride film 13 and support substrate 11 may be bondedto each other in either the direct manner or the indirect manner asdescribed above, in order to increase the joint strength, group IIInitride film 13 and support substrate 11 are preferably bonded to eachother in an indirect manner with joint film 12 interposed therebetweenas shown in FIG. 4 (A). Details will be described below.

Referring to FIG. 4 (A), the step of bonding group III nitride film 13and support substrate 11 to each other in an indirect manner with jointfilm 12 interposed therebetween includes: the sub step of forming jointfilm 12 a on main surface 11 m of support substrate 11 (FIG. 4 (A1));the sub step of forming joint film 12 b on main surface 13 n of groupIII nitride film 13 (FIG. 4 (A2)); and the sub step of bonding jointfilm 12 a formed on main surface 11 m of support substrate 11 and jointfilm 12 b formed on main surface 13 n of group III nitride film 13 toeach other (FIG. 4 (A3)). These sub steps are performed to join andthereby integrate joint film 12 a and joint film 12 b bonded to eachother into joint film 12, and support substrate 11 and group III nitridefilm 13 are joined to each other with joint film 12 interposedtherebetween.

As to the method for manufacturing group III nitride composite substrate1 in the present embodiment as well, due to the presence of surfaceroughness of respective main surfaces of support substrate 11, group IIInitride film 13, and joint films 12, 12 a, 12 b, main-surface throughhole 13 h of group III nitride film 13, non-uniformity of bonding, andthe like, there are formed, in joint interface 100, joined region 100 bwhere group III nitride film 13 and support substrate 11 are joined toeach other in either the direct manner or the indirect manner, andnon-joined region 100 n where group III nitride film 13 and supportsubstrate 11 are not joined to each other in either the direct manner orthe indirect manner. The description of joined region 100 b andnon-joined region 100 n is given above.

The method for forming joint films 12 a, 12 b is not particularlylimited. In order to reduce the cost of forming the joint film, it ispreferable to perform sputtering, vapor deposition, CVD (Chemical VaporDeposition), or the like. The method for bonding support substrate 11and group III nitride film 13 to each other by bonding joint film 12 aand joint film 12 b to each other is not particularly limited, andpreferred methods are direct bonding method, surface activated bondingmethod, high-pressure bonding method, high vacuum bonding method, andthe like as described above.

<Step of Reducing Thickness of at Least One of Group III Nitride Filmand Support Substrate>

Referring to FIG. 4 (B), the method for manufacturing group III nitridecomposite substrate 1 in the present embodiment includes the step ofsubsequently reducing the thickness of at least one of group III nitridefilm 13 and support substrate 11 bonded to each other. Here, the methodfor reducing the thickness of at least one of group III nitride film 13and support substrate 11 is not particularly limited, and may forexample be the method according to which the substrate whose thicknessis to be reduced is cut in parallel with the main surface, the methodaccording to which the main surface of the substrate whose thickness isto be reduced is ground and/or polished, the method according to whichthe main surface of the substrate whose thickness is to be reduced isetched, the method by means of a laser, or the like. The method by meansof a laser is a method according to which a laser beam is applied sothat the focus of the laser beam is located at a predetermined depthfrom the main surface of the substrate. In the case of the method bymeans of a laser, the chemical composition of the region of the positionat a predetermined depth from the main surface of the substrate wherethe laser beam is focused is changed, and accordingly the substrate canbe divided at this region. Since the method by means of a laser changesthe chemical composition of a region of a position at a predetermineddepth from the main surface of the substrate and does not change thechemical composition of the region other than the above-referencedregion, femtosecond laser, picosecond laser, or the like is preferablyused.

Third Embodiment Method for Manufacturing Group III NitrideSemiconductor Device

Referring to FIG. 5, a method for manufacturing a group III nitridesemiconductor device 2 which is still another embodiment of the presentinvention includes the steps of: preparing group III nitride compositesubstrate 1, and growing at least one group III nitride layer 20 on mainsurface 13 m on the side of group III nitride film 13 of group IIInitride composite substrate 1. The method for manufacturing group IIInitride semiconductor device 2 in the present embodiment can include theabove steps to thereby manufacture group III nitride semiconductordevices with a high yield.

<Step of Preparing Group III Nitride Composite Substrate>

The method for manufacturing group III nitride semiconductor device 2 inthe present embodiment includes the step of firstly preparing group IIInitride composite substrate 1. This step of preparing group III nitridecomposite substrate 1 is similar to the steps in the method formanufacturing group III nitride composite substrate 1 in the secondembodiment.

<Step of Growing Group III Nitride Layer>

The method for manufacturing group III nitride semiconductor device 2 inthe present embodiment includes the step of subsequently growing atleast one group III nitride layer 20 on main surface 13 m on the side ofgroup III nitride film 13 of group III nitride composite substrate 1.

The method for growing group III nitride layer 20 is not particularlylimited. In order to grow group III nitride layer 20 of a high crystalquality, preferred methods include vapor phase methods such as MOCVD(Metal Organic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy),HVPE (Hydride Vapor Phase Epitaxy), sublimation method, and the like,and liquid phase methods such as high nitrogen pressure solution method,flux method, and the like.

The structure of group III nitride layer 20 to be grown varies dependingon the type and the function of the group III nitride semiconductordevice. In the case where group III nitride semiconductor device 2 is alight-emitting device, group III nitride layer 20 may be formed on mainsurface 13 m on the side of group III nitride film 13 of group IIInitride composite substrate 1 by growing a first-conductivity-type GaNlayer 21, a first-conductivity-type Al_(s)Ga_(1-s)N layer 22 (here, smeets 0<s<1), a light-emitting layer 23, a second-conductivity-typeAl_(t)Ga_(1-t)N layer 24 (here, t meets 0<t<1), and asecond-conductivity-type GaN layer 25, in this order.

The method for manufacturing group III nitride semiconductor device 2 inthe present embodiment may include the step of subsequently formingelectrodes (a first electrode and a second electrode). In group IIInitride layer 20, second-conductivity-type GaN layer 25,second-conductivity-type Al_(t)Ga_(1-t)N layer 24, light-emitting layer23, and first-conductivity-type Al_(s)Ga_(1-s)N layer 22 may each bepartially removed by mesa etching to expose a part offirst-conductivity-type GaN layer 21. On the exposed main surface offirst-conductivity-type GaN layer 21, first electrode 31 may be formed.On the exposed main surface of the second-conductivity-type GaN layer,second electrode 32 may be formed. The method for forming firstelectrode 31 and second electrode 32 is not particularly limited, andmay be CVD (Chemical Vapor Deposition), sputtering, vapor deposition, orthe like EXAMPLES

Example 1 1. Fabrication of Group III Nitride Composite Substrate

(1) Bonding Group III Nitride Film and Support Substrate

Referring to FIG. 4 (A), group III nitride film 13 and support substrate11 were bonded to each other in the following way.

Referring to FIG. 4 (A1), support substrate 11 was prepared.Specifically, three mullite (Al₂O₃—SiO₂) substrates each having two mainsurfaces both polished into a mirror surface (flat surface having anarithmetic mean surface roughness Ra defined under JIS B0601 of 1 nm orless, this definition will be applied hereinafter) and having a diameterof 10.0 cm and a thickness of 500 μm were prepared. On main surface 11 mof this support substrate 11, an SiO₂ film of 1 μm in thickness wasformed as joint film 12 a by CVD.

Referring to FIG. 4 (A2), group III nitride film 13 was prepared.Specifically three different GaN films A to C each having two mainsurfaces both polished into a mirror surface and having a diameter of10.0 cm and a thickness of 400 μm were prepared. GaN film A was notactively doped with a conductivity improver impurity. GaN film B wasdoped with O (oxygen) atoms which serve as a conductivity improverimpurity at a concentration of 7×10¹⁷ cm⁻³. GaN film C was doped with O(oxygen) atom which serve as a conductivity improver impurity at aconcentration of 1.6×10¹⁸ cm⁻³.

On main surface 13 n which was a (000-1) plane corresponding to an Natomic plane of group III nitride film 13, an SiO₂ film of 1 μm inthickness was formed as joint film 12 b by CVD.

Referring next to FIG. 4 (A3), each of main surface 12 am of joint film12 a formed on support substrate 11 and main surface 12 bn of joint film12 b formed on group III nitride film 13 was polished into a mirrorsurface, and thereafter main surface 12 am of joint film 12 a and mainsurface 12 bn of joint film 12 b were bonded to each other. Annealingwas performed by raising the temperature to 700° C. in an N₂ gas(nitrogen gas) atmosphere to thereby increase the joint strength.

The three different bonded substrates in which they were thus bonded toeach other were each cut by dicing into eight bonded substrate pieceshaving a main surface of 20 mm×20 mm (square of 20 mm×20 mm, which willalso be applied hereinafter).

(2) Adjustment of Thickness of Group III Nitride Film

Referring to FIG. 4 (B), the thickness of group III nitride film 13 ofthe eight bonded substrate pieces obtained from each of the threedifferent bonded substrates was adjusted by polishing the main surfaceso that respective group III nitride films were 5 μm, 8 μm, 10 μm, 20μm, 50 μm, 100 μm, 200 μm, and 500 μm. Thus, eight group III nitridecomposite substrates were obtained.

2. Measurement of Sheet Resistance of Group III-Nitride-Film-Side MainSurface of Group III Nitride Composite Substrate

For the 24 group III nitride composite substrates 1 obtained from thethree different bonded substrates produced in the above-described manner(eight group III nitride composite substrates per each of the threedifferent bonded substrates), the sheet resistance of groupIII-nitride-film 13-side main surface 13 m was measured by thefour-terminal method. The results are summarized in Table 1.

TABLE 1 group III nitride film sheet resistance type thickness (μm)(Ω/sq) GaN film A 5 403.6 8 250.8 10 196.1 20 101.6 50 41.5 100 19.8 2009.7 500 3.8 GaN film B 5 157.7 8 101.3 10 78.9 20 39.2 50 18.1 100 7.4200 3.7 500 1.3 GaN film C 5 40.7 8 24.5 10 19.4 20 9.7 50 3.6 100 1.7200 0.9 500 0.3

3. Fabrication of Group III Nitride Semiconductor Device

Referring to FIG. 5, on main surface 13 m which was a (0001) planecorresponding to a group III atomic plane of group III nitride film 13in group III nitride composite substrate 1, group III nitride layer 20was grown by MOCVD. Specifically, an n-type GaN layer having a thicknessof 5 μm which was first-conductivity-type GaN layer 21, an n-typeAl_(0.05)Ga_(0.95)N layer having a thickness of 0.5 μm which wasfirst-conductivity-type Al_(s)Ga_(1-s)N layer 22 (here, s meets 0<s<1),an MQW (Multiple Quantum Well) structure layer having a thickness of 100nm and made up of six cycles of In_(0.35)Ga_(0.85)N layer andIn_(0.05)Ga_(0.95)N layer which was light-emitting layer 23, a p-typeAl_(0.02)Ga_(0.80)N layer having a thickness of 20 nm which wassecond-conductivity-type Al_(t)Ga_(1-t)N layer 24 (here, t meets 0<t<1),and a p-type GaN layer having a thickness of 0.15 μm which wassecond-conductivity-type GaN layer 25 were grown in this order.

Next, second-conductivity-type GaN layer 25, second-conductivity-typeAl_(t)Ga_(1-t)N layer 24, light-emitting layer 23, andfirst-conductivity-type Al_(s)Ga_(1-s)N layer 22 were each partiallyremoved by mesa etching to expose a part of first-conductivity-type GaNlayer 21.

Next, on the exposed main surface of first-conductivity-type GaN layer21, an n-side electrode was formed as first electrode 31 by electronbeam vapor deposition. In addition, on the exposed main surface ofsecond-conductivity-type GaN layer 25, a p-side electrode was formed assecond electrode 32 by electron beam vapor deposition. In this way, 24group III nitride semiconductor devices were fabricated.

A reference group III nitride semiconductor device was prepared in thefollowing way. On main surface 13 m of the GaN substrate that was a(0001) plane corresponding to a Ga atomic plane, group III nitride layer20 of the same structure as the above-described one was formed in asimilar manner to the above-described one. The GaN substrate had the twomain surfaces both polished into a mirror surface and having a size of20 mm×20 mm and a thickness of 500 μm, and was doped with a conductivityimprover impurity, namely O (oxygen) atoms, at a concentration of1.6×10¹⁸ cm⁻³.

4. Measurement of Emission Intensity of Group III Nitride SemiconductorDevice

The emission intensity of the 24 group III nitride semiconductor devicesand the reference group III nitride semiconductor device obtained asdescribed above was measured in the following way. Current of 80 mA wasapplied to the group III nitride semiconductor device. The emissionintensity at a peak wavelength of 450 nm of the emission spectrum atthis time was measured by the EL (Electro Luminescence) method. Theratio of the emission intensity of each of the 24 group III nitridesemiconductor devices, relative to the emission intensity of thereference group III nitride semiconductor device, was calculated for useas a relative emission intensity. A light-emitting device having a groupIII nitride composite substrate including GaN film A of 5 μm inthickness and having a sheet resistance of 403.6 Ω/sq, and alight-emitting device having a group III nitride composite substrateincluding GaN film A of 8 μm in thickness and having a sheet resistanceof 250.8 Ω/sq each had a low relative emission intensity of 0.01 orless. The relative emission intensity of light-emitting devices havingthe other group III nitride composite substrates had a higher relativeemission intensity of 0.1 or more.

Example 2

As group ITT nitride films 13, 20 GaN films having a diameter of twoinches (5.08 cm) and a thickness of 500 μm, doped with O (oxygen) atomswhich were a conductivity improver impurity at a concentration of 7×10¹⁷cm⁻³, and having two main surfaces both polished into a mirror surfacewere prepared.

Of these GaN films, ten GaN films were polished so that the thicknesswas reduced to 200 μm. As a result, breakage and/or cracks occurred toseven GaN films out of the ten GaN films.

To the remaining ten GaN films each, a mullite substrate having adiameter of two inches (5.08 cm) and a thickness of 400 μm was bondedthrough a similar procedure to Example 1, and thereafter the bonded GaNfilm was polished so that the thickness was reduced to 200 μm. Then, nobreakage and/or cracks occurred to all of the ten GaN films.

Example 3

Six group III nitride composite substrates 1 were fabricated through asimilar procedure to Example 1, except that a GaN film having a diameterof two inches (5.08 cm) and a thickness of 500 μm, doped with noconductivity improver impurity, and having two main surfaces bothpolished into a mirror surface was used as group III nitride film 13,and that a watermark was formed on the surface to be bonded to therebychange the area of the bonded region. Here, the watermark is a dry stainformed as follows. As moisture in a portion of a surface where animpurity is present due to cleaning contamination or the like is dried,the portion is dried with the impurity aggregated, and accordingly thedry stain is formed. The watermark can be formed by wetting a surfacewhere the watermark is to be formed, with low-purity water rather thanultrapure water and drying it to thereby form the watermark in theportion wet with the low-impurity water.

For the obtained six group III nitride composite substrates 1, the areaof joined region 100 b was measured with a surface detect inspectionapparatus using a laser. As a result, respective areas of the joinedregions of the substrates relative to the area of main surface 1 m were34%, 57%, 64%, 70%, 87%, and 95%. Of these group III nitride compositesubstrates, group III nitride composite substrates 1 in which the areaof joined region 100 b was 70% or more relative to the area of mainsurface 1 m had non-joined partial regions constituting non-joinedregion 100 n that all had a maximum size in radial direction of lessthan 20 mm. As for group III nitride composite substrates 1 m which thearea of joined region 100 b was 64% or less relative to the area of mainsurface 1 m, some non-joined partial regions constituting non-joinedregion 100 n had a maximum size in radial direction of 20 mm or more.Moreover, group III nitride composite substrates 1 m which the area ofjoined region 100 b was 64% or less relative to the area of main surface1 m were heated to 700° C. As a result, group III nitride film 13 waspeeled off during the heating, from support substrate 11 from thenon-joined partial region with a maximum size in radial direction of 20mm or more.

Example 4

Three group III nitride composite substrates 1 were fabricated through asimilar procedure to Example 1, except that: a GaN film having adiameter of two inches (5.08 cm) and a thickness of 500 μm, doped withno conductivity improver impurity, and having two main surfaces bothpolished into a mirror surface was used as group III nitride film 13; apolycrystalline Mo substrate having a diameter of two inches (5.08 cm)and a thickness of 500 μm and having two main surfaces both polishedinto a mirror surface was used as support substrate 11; the surfaces tobe bonded to each other were cleaned in the process of bonding tothereby reduce generation of non-joined regions; and a water mark havinga diameter on the order of 15 mm was formed on the surface to be bondedto thereby form a non-joined region at a predetermined position of thegroup III nitride composite substrate.

For the obtained three group III nitride composite substrates 1, theposition of the non-joined region was measured with an ultrasonicmicroscope. As a result, in one group III nitride composite substrate,there was formed the non-joined region including a non-joined partialregion having a maximum size in radial direction on the order of 10 mmand abutting on the perimeter of the main surface. In another group IIInitride composite substrate, there was formed the non-joined regionincluding only a non-joined partial region having a maximum size inradial direction on the order of 10 mm and failing to abut on theperimeter of the main surface. In the remaining one group III nitridecomposite substrate, the area of the non-joined region was 83% relativeto the area of the main surface.

The obtained three group III nitride composite substrates were heated to700° C. As a result, the group III nitride composite substrate in whichthere was formed the non-joined region including the non-joined partialregion which had a maximum size in radial direction on the order of 10mm and abutted on the perimeter of the main surface was broken duringthe heating. The remaining group III nitride composite substrates werenot broken.

Example 5

As group III nitride films, eight GaN films whose two main surfaces wereboth polished into a mirror surface, main surface was 20 mm×20 mm, andthickness was 300 μm were prepared. The eight GaN films were dividedinto four sets each including two GaN films, and main-surface throughholes having a diameter on the order of 50 μm were formed so that thearea of the main-surface through holes relative to the area of the mainsurface were 5%, 10%, 20%, and 30%, respectively in these four sets.

Moreover, eight substrates were prepared in which the support substratewas a sapphire substrate whose main surface was 20 mm×20 mm andthickness was 400 μm and an SiO₂ film was formed as the joint film onthe main surface of the sapphire substrate. The main surface of thejoint film was polished into a mirror surface. After this, four of theeight substrates were cleaned multiple times in an aqueous solutioncontaining 37% by mass of HCl (hydrochloric acid) and 30% by mass ofH₂O₂ (hydrogen peroxide), to thereby remove an impurity containing metalon the surface to be bonded. The remaining four substrates were cleanedwith IPA (isopropyl alcohol).

The four group III nitride films in which respective areas of themain-surface through holes were 5%, 10%, 20%, and 30% relative to thearea of the main surface, and the four support substrates in which thesurface of the joint film to be bonded was cleaned with HCl and H₂O₂were bonded to each other under similar conditions to Example 1.Further, the four group III nitride films in which respective areas ofthe main-surface through holes were 5%, 10%, 20%, and 30% relative tothe area of the main surface and the four support substrates in whichthe surface of the joint film to be bonded was cleaned with IPA werebonded to each other under similar conditions to Example 1.

After they were thus bonded to each other, a tensile test was performedin the following way. The tensile test was performed by attaching a jigof 10 mm×10 mm with an epoxy adhesive to the front and rear sides of thegroup III nitride composite substrate and pulling both ends of the jig.The results are summarized in Table 2. In Table 2, the compositesubstrates on which fracture occurred between the epoxy adhesive and thejig exhibit a higher joint strength than those in which the group ITTnitride film and the joint film are separated from each other alongtheir interface.

TABLE 2 area (%) of main- surface through method for hole in group IIIcleaning nitride film surface to relative to area of region of fractureand form of be bonded main surface fracture under tensile test cleaningwith 5 fracture along interface between HCl and H₂O₂ epoxy adhesive andjig 10 separation along interface between group III nitride film andjoint film 20 separation along interface between group III nitride filmand joint film 30 separation along interface between group III nitridefilm and joint film cleaning with 5 fracture along interface between IPAepoxy adhesive and jig 10 fracture along interface between epoxyadhesive and jig 20 separation along interface between group III nitridefilm and joint film 30 separation along interface between group IIInitride film and joint film

Referring to Table 2, the composite substrates which were cleaned withIPA had a higher joint strength where the area of the main-surfacethrough hole of the group III nitride film was 10% or less relative tothe area of the main surface. In addition, the concentration of animpurity containing metal on the surface to be bonded of the joint filmcleaned with HCl and H₂O₂ and that of the joint film cleaned with IPAwere measured by TXRF (total reflection x-ray fluorescence) analysis.Those cleaned with HCl and H₂O₂ had an Fe concentration and an Niconcentration which were both 1×10¹⁰ cm⁻² or less. Those cleaned withIPA had an Fe concentration of 5.7×10¹¹ cm⁻² and an Ni concentration of1.5×10¹¹ cm⁻².

Comparative Example 1

As the group III nitride film, one GaN film having a diameter of twoinches (5.08 cm) and a thickness of 400 μm, doped with O (oxygen) atomsat a concentration of 6×10¹⁷ cm⁻³ which was a conductivity improverimpurity, and having two main surfaces both polished into a mirrorsurface was prepared. In a main surface, which was an N atomic surfaceof the group III nitride film, H (hydrogen) ions were implanted to aposition at a depth of about 0.7 μm from the main surface. The H ionswere implanted with an acceleration voltage of 100 keV and a dose of5×10¹⁷ cm⁻². The main surface, where the H ions were implanted, of thegroup III nitride film implanted with the H ions, and a mullitesubstrate having a diameter of two inches (5.08 cm) and a thickness of500 μm, were bonded to each other and annealed through a similarprocedure to Example 1. Accordingly, the group III nitride film wasseparated along the region where the H ions were implanted, so that agroup III nitride composite substrate in which a group III nitride filmof 0.3 μm in thickness is formed on the mullite substrate was obtained.The sheet resistance of the obtained group III nitride compositesubstrate was 1×10¹⁰ Ω/sq or more.

Comparative Example 2

A group III nitride composite substrate having a group III nitride filmof 0.3 μm in thickness on an Mo substrate was produced through a similarprocedure to Comparative Example 1, except that the Mo substrate havinga diameter of two inches (5.08 cm) and a thickness of 300 μm and havingtwo main surfaces both polished into a mirror surface was used as thesupport substrate, and that the dose of H ions was 3.5×10¹⁷ cm⁻². Theobtained group III nitride composite substrate was further heat-treatedat 800° C. for three hours in an N₂ gas (nitrogen gas) atmosphere.Accordingly, the sheet resistance was reduced to 6700 Ω/sq in most ofthe region, while the sheet resistance was still a high resistance of35000 Ω/sq in a part of the region.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the scopeof the present invention being interpreted by the terms of the appendedclaims.

1-8. (canceled) 9: A method for manufacturing a group III nitridecomposite substrate comprising a group III nitride film and a supportsubstrate formed from a material different in chemical composition fromsaid group III nitride film, the method comprising the steps of: bondingsaid group III nitride film and said support substrate to each other inone of a direct manner and an indirect manner; and reducing thethickness of at least one of said group III nitride film and saidsupport substrate bonded to each other, said group III nitride filmhaving a thickness of 10 μm or more, wherein a sheet resistance of agroup III-nitride-film-side main surface of said group III nitridecomposite substrate is 200 Ω/sq or less, said groupIII-nitride-film-side main surface being a main surface common to saidgroup III nitride composite substrate and said group III nitride film.10: A method for manufacturing a group III nitride semiconductor deviceusing a group III nitride composite substrate comprising a group IIInitride film and a support substrate formed from a material different inchemical composition from said group III nitride film, the methodcomprising the steps of: preparing said group III nitride compositesubstrate, said group III nitride film being joined to said supportsubstrate in one of a direct manner and an indirect manner; and growingat least one group III nitride layer on the main surface of the side ofsaid group III nitride film of said group III nitride compositesubstrate, said group III nitride film having a thickness of 10 μm ormore, wherein a sheet resistance of a group III-nitride-film-side mainsurface of said group III nitride composite substrate being 200 Ω/sq orless, said group III-nitride-film-side main surface being a main surfacecommon to said group III nitride composite substrate and said group IIInitride film.